Pentatricopeptide repeat (PPR) proteins are RNA-binding proteins that are widely distributed in plants. They contain 2 to 30 repeating units of ~35-amino acid PPR motifs. They are known to play important roles in RNA processing, RNA editing, and translational regulation. Recent studies on the RNA recognition mode of PPR proteins revealed that one PPR motif interacts with one nucleotide. In addition, it was revealed that amino acids at three specific positions in a single motif serve to specify its binding base. Thus, mutation of these amino acids can cause a modification of the binding specificity of PPR motifs. Indeed, the engineered PPR motifs fused with various effector domains are shown to bind to and manipulate RNAs in a controlled manner. In this review, we summarize the recent progress in structural studies on PPR motifs. We focus on their RNA recognition mode and discuss the potentials of PPR as novel, versatile tools for RNA manipulation.
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